1. Field of the Invention
The present invention relates to a device which calculates the estimated vehicle speed of a vehicle for use in wheel behavior control, including antilock brake control, and more particularly, to a device for estimating the direction of travel of the vehicle based on the output of a wheel speed determination device and a gravity-type accelerometer.
2. Description of the Prior Art
Antilocks brake control devices and other wheel behavior control devices base their control on estimated values such as the extent of wheel slippage or spinning based on a calculated vehicle speed, and estimate the coefficient of friction between a tire and the road surface. The coefficient of friction between the tire and road is normally estimated based on a estimated value of vehicle acceleration during control, and the vehicle acceleration is estimated based on the estimated vehicle speed. Therefore, the performance of the wheel behavior control device is greatly dependent upon the precision of vehicle speed estimation.
However, if the vehicle speed is estimated from the wheel speed alone, the estimating precision is reduced significantly when the tires slip or spin badly.
Therefore, if the vehicle speed and vehicle acceleration are estimated based on both the wheel speed and the acceleration obtained from a reliable accelerometer, the estimating precision can be significantly increased. A typical type of accelerometer is a gravity-type accelerometer.
As illustrated in FIG. 6, the gravity-type accelerometer detects acceleration by converting the displacement of a weight F caused by acceleration to an electrical signal using resistors, piezoelectric elements, differential transformers, and other devices. The gravity-type accelerometer measures an acceleration as a signed value, while the wheel speed speedometer usually measures a speed as an unsigned value, i.e. an absolute value.
Therefore, an estimated value of vehicle acceleration calculated in accordance with a wheel speed value produced wheel speed speedometers will not have the same sign as an acceleration value obtained from a gravity-type accelerometer, when the vehicle is travelling in a reverse direction. If the direction of vehicle travel is not identified by some means, it will not be possible to control wheel behavior when the vehicle travels in a reverse direction.
Although it is possible to use the sign (positive or negative) output by a gravity-type accelerometer, the sign may not be accurate, due to e.g., inaccuracies in installation of the accelerometer, electrical drift and gain changes, and other relatively long-term variations. Furthermore, it is also affected by the slope of a road where the vehicle is driven on. In these circumstances output the sign (polarity) cannot necessarily be used as the true sign of acceleration.
Therefore, when using a gravity-type accelerometer, it is extremely important to solve these problems, including the problem of the slope of the road. In the applicant's previously filed related application (Ser. No. 07/811,942) corresponding to Japanese Patent application H2-406743, a zero-point correction device for a gravity-type accelerometer is introduced. By the use of the zero-point correction device as disclosed in the related application, it is possible to estimate the sign of the true acceleration with a certain degree of reliability using the sign of the value output by the gravity-type accelerometer after zero-point correction.
In the above related application, however, the zero-point correction is carried out under the assumption that the direction of vehicle travel is correctly detected. If the zero-point correction is carried out based on a false assumption of the direction of vehicle travel, the correction will contribute to an unexpected result which is much worse than without correction.
Therefore, when a gravity-type accelerometer is used it is very important to correctly recognize the direction of vehicle travel.